This invention relates to the investigation of objects with ultrasound, and, more particularly, to improvements in the efficient coupling of ultrasonic energy between a transducer and a transmission medium. The invention is especially useful in ultrasonic imaging systems.
In recent years ultrasonic techniques have become more prevalent in clinical diagnosis. Such techniques have been utilized for some time in the field of obstetrics, neurology and cardiology, and are becoming increasingly important in the visualization of subcutaneous blood vessels including imaging of smaller blood vessels.
Various fundamental factors have given rise to the increased use of ultrasonic techniques. Ultrasound differs from other forms of radiation in its interaction with living systems in that it has the nature of a mechanical wave. Accordingly, information is available from its use which is of a different nature than that obtained by other methods and it is found to be complementary to other diagnostic methods, such as those employing X-rays. Also, the risk of tissue damage using ultrasound appears to be much less than the apparent risk associated with ionizing radiations such as X-rays.
The majority of diagnostic techniques using ultrasound are based on the pulse-echo method wherein pulses of ultrasonic energy are periodically generated by a suitable piezoelectric transducer such as lead zirconate-titanate ceramic. Each short pulse of ultrasonic energy is focused to a narrow beam which is transmitted into the patient's body wherein it eventually encounters interfaces between various different structures of the body. Where there is a characteristic impedance mismatch at an interface, a portion of the ultrasonic energy is reflected at the boundary back toward the transducer. After generation of the pulse, the transducer operates in a "listening" mode wherein it converts received reflected energy or "echoes" from the body back into electrical signals. The time of arrival of these echoes depends on the ranges of the interfaces encountered and the propagation velocity of the ultrasound. Also, the amplitude of the echo is indicative of the reflection properties of the interface and, accordingly, of the nature of the characteristic structures forming the interface.
There are various ways in which the information in the received echoes can be usefully presented. In one common technique, the electrical signals representative of detected echoes are amplified and applied to the vertical deflection plates of a cathode ray display. The output of a time-base generator is applied to the horizontal deflection plates. Continuous repetition of the pulse/echo process in synchronism with the time-base signals produces a continuous display, called an "A-scan", in which time is proportional to range, and deflections in the vertical direction represent the presence of interfaces. The height of these vertical deflections is representative of echo strength.
Another common form of display in the so-called "B-scan" wherein the echo information is of a form more similar to conventional television display; i.e., the received echo signals are utilized to modulate the brightness of the display at each point scanned. This type of display is found especially useful when the ultrasonic energy is scanned transverse the body so that individual "ranging" information yields individual scanlines on the display, and successive transverse positions are utilized to obtain successive scanlines on the display. This type of technique yields a cross-sectional picture in the plane of the scan, and the resultant display can be viewed directly or recorded photographically or on magnetic tape. The transverse scan of the beam may be achieved by a reflector which is scanned mechanically over a desired angle.
In systems of the type described, it is desirable to couple, with maximum efficiency, ultrasound power from the transducer into the adjacent transmission medium. Typically, the ultimate transmission medium is a fluid such as water, although an ultrasound lens, formed for example of plastic, may be disposed between the transducer and the transmission fluid. In either case, it is also desirable that the wave energy be efficiently transferred to the transmission medium over a relatively wide bandwidth, thereby enhancing range resolution by virtue of return echo signals having a relatively wide bandwidth. Considerations of sufficient power transfer from the transmission medium to the transducer also come into play in the same fashion (during "listening" for return echo signals). Unfortunately, the acoustic impedance of the piezoelectric crystals employed as transducers is quite different from the acoustic impedance of typical transmission medium, be it water or a focusing lens (such as a plastic focusing lens). Conventional matching techniques can be employed to provide matching between the transducer crystal and the transmission medium, this matching typically being provided at the resonant frequency of the crystal. For example, the matching can be of the type wherein one employs a quarter-wave matching section whose acoustic impedance is the geometric mean between that of the piezoelectric crystal and that of the transmission medium. However, such techniques generally result in a relatively narrow bandwidth power spectrum of the signal transmitted into the transmission medium. The consequence is a relatively narrow bandwidth return signal which can result in a relatively poor resolution image.
It has been further suggested that the effective bandwidth of operation could be broadened by using a backing material for the transducer which is highly lossy and has approximately the same acoustic impedance as the transducer crystal. However, this technique has been found to be of limited effectiveness, and it is difficult to find materials having the appropriate physical properties along with the stated loss and acoustic impedance characteristics.
It is among the objects of this invention to provide an improved matching region for efficiently coupling relatively broadband ultrasonic energy to a transmission medium, these improvements being responsive to the type of prior art problems just set forth.